1. Field of the Invention
This invention relates to thermostatic steam traps and in particular to such a trap which can operate in two different modes. In this regard, reference should be made to U.S. Pat. Nos. 4,549,691 and 4,560,105 (Jiandani) which indicate that the purpose of a steam trap is to vent air, or other noncondensables, and condensate from a steam pressure system with a minimum loss of steam. As further indicated in these patents, there are several different types of trap, of which some are better suited to particular uses than others. Discussions about this point contained in U.S. Pat. Nos. 4,549,691 and 4,560,105 can be summarised as follows. It may be necessary to utilise more than one type of trap in a single process plant. For example, in average chemical process plants and oil refineries there are two important uses for steam traps. They are used in connection with steam tracing of product lines, which must be heated to keep their contents at desired temperature and/or viscosity. They are also used for draining condensate from steam mains. These two uses, however, require different operating characteristics. In the draining of condensate from steam mains, the condensate is removed at approximately saturation temperature even though it contains sufficient sensible heat. This is so because the presence of condensate is undesirable and under some conditions, hazardous. In the draining of steam tracing lines, however, condensate is removed at temperatures well below saturation in order to achieve maximum utilisation of sensible heat. Plants often stock thermostatic traps for the first use, and so-called liquid expansion traps for the second use, and it will be appreciated that there is a need for a single trap able to perform functions traditional to both thermostatic traps and liquid expansion traps. By this it is not meant merely to have a common housing with interchangeable internals but, rather, a trap which is truly operable in two modes, in one of which the trap is to discharge condensate at a temperature close to saturated steam temperature, while in the other the discharge is to be at a temperature significantly below saturated steam temperature.
2. Description of the Prior Art
Attention is directed to the detailed discussions of prior art contained in U.S. Pat. Nos. 4,549,691 and 4,560,105, and to the prior art listed under "References Cited" in each of these patents. The steam trap forming the subject of 4 560 105 is well illustrated in FIG. 2 of the patent where it is shown as being a bellows-actuated thermostatic steam trap (A) having a bellows (41) carrying a valve member (45) of a valve (45/22) for opening and closing the trap, the bellows being disposed between connections (20/21 and 30/31) which are identical to one another so as to be selectively connectable in a steam flow line with the valve (45/22) disposed either downstream of, or upstream of, the bellows (41). The trap operates, when connected with the valve disposed downstream of the bellows, in a first mode in which, in normal operation, the trap discharges condensate at a temperature close to saturated steam temperature. When connected with the valve disposed upstream of the bellows, the valve operates in a second mode in which in normal operation condensate is discharged at a temperature significantly below steam temperature. One or other of these modes can be simply obtained merely by selecting which way round the trap is connected in a flow line. The bellows (41) is disposed in an internal chamber (40) of a housing (11/12), the bellows having opposed ends (42, 43) interconnected by an accordion side wall (44). The bellows contains a volatile liquid whose saturation curve closely parallels but is few degrees below that for saturated steam.
When the trap of 4 560 105 is to operate in its first mode, in which hot condensate is discharged until the appearance of steam, the trap is connected in a steam system such that condensate flows into the chamber containing the bellows before reaching the valve of the trap. Thus the environment within the chamber is reflective of pressure and temperature conditions immediately upstream of the trap. When cold, the trap is wide open, freely discharging non-condensables and cool condensate. As the condensate temperature increases, the liquid in the bellows evaporates and generates a significant vapour pressure. When the condensate temperature reaches a few degrees Farenheit (e.g. 10 degrees) below saturated steam temperature, the vapour pressure within the bellows equals the pressure of the condensate in the surrounding chamber. As the condensate temperature increases farther and approaches that of steam, the internal vapour pressure exceeds the external pressure, causing the bellows to expand, driving the valve member of the valve toward its seat. If steam temperature is reached the valve member is driven tightly into its seat, closing the trap. As the condensate surrounding the bellows cools, the vapourised liquid within the bellows condenses, reducing the internal pressure. The bellows contracts, opening the trap for discharge. Thus, the thermostatic trap when operating in its first mode discharges condensate at close to saturation temperature.
If it is desired that the trap of U.S. Pat. No. 4,560,105 should operate in its second mode, in which there is modulation of a stream of condensate having a temperature significantly below the equilibrium temperature for saturated steam, the valve is connected into a steam system in reversed configuration, as compared with the first mode, so that the valve is interposed between the chamber and the steam system. As a result, once the trap is operating, conditions in the chamber, in particular the pressure in the chamber, are ambient conditions (e.g. atmospheric) rather than system conditions. In this second mode, upon start up, the trap is open, freely discharging noncondensables and cool condensate until the condensate reaches a predetermined temperature below 212 degrees F. (e.g. anywhere from approximately 6 to 20 degrees, depending upon pressure at the trap inlet). As the hot condensate flows over the bellows the liquid in the bellows boils, exerting a pressure which expands the bellows, pushing the valve toward its seat, and thus throttling flow. Because the surrounding pressure in the chamber is always atmospheric, the bellows always expands at the predetermined temperature below 212 degrees F. irrespective of the condensate line pressure. Flow through the valve of condensate approaching 212 degrees F., may cause the valve momentarily to close but, because the bellows is downstream of the valve, the bellows is then cut off from its heat source and the valve begins to open again. Thus there is a continuous search for equilibrium which results in a modulated discharge from the trap (i.e. increased and reduced flow but nearly always some flow) once initial heat-up has occurred. This is different from the first mode of operation and is attributable to the differences in environment in the chamber. The valve member movement is relatively small, of the order of small fractions of an inch, but such relatively small movement is enough to produce the desired results.
A drawback with the steam traps of U.S. Pat. Nos. 4,549,691 and 4,560,105 is that if the bellows should fail, there will be equalisation of pressure as between the interior and the exterior of the bellows with the result that the valve closes. In other words, these traps fail closed and this is undesirable as in such circumstances there is a tendency for process temperature to rise.
Attention is also drawn to U.S. Pat. No. 4,295,605 (Clayton et al), to the prior art listed in this patent under "References Cited", and, in particular, to the thermostatic element of the balanced pressure thermostatic steam trap shown in FIG. 2B of No. 4,295,605. This element comprises a housing (6) within which there is disposed a multi-diaphragm arrangement (2B) consisting of two diaphragms (23, 24) sealed to one another at their peripheries so as to define an interior void (10) that opens from the housing (6) through an aperture (11) in the bottom wall (7) of the housing (6). The lower diaphragm (23) is fast with this bottom wall, whereas the upper diaphragm (24) is free to move up and down and carries for moving with it a valve member (13) that projects through the aperture (11). The interior of the housing (6), outside the multi-diaphragm arrangement (2B), is filled with volatile fluid (15). As in the case of each of Nos. 4,549,691 and 4,560,105, if the multi-diaphragm arrangement (17) fails, there is equalisation of pressure across the movable diaphragm (24) and this trap also fails closed.